Support a data compression in buffer plugins

[ganmacs]

Buffer plugins have data as raw String object.
This way consumes many memory resource and bandwidth of the network.
To reduce this wasteful consumption, I propose that support a data compression in buffer plugins.

Tasks

• Add compress true in buffer plugin configuration.
• Compress a chunk by using zlib in concat
• Decompress a chunk in write_to

[tagomoris]

It looks better to do decompression in #open with optional argument not to do it forcedly (to get compressed binary for any purpose, including compressed data transferring over network).
But let me see how your implementation goes.

[ganmacs]

I think implementing decompression code in #open doesn't work well in MemoryChunk class.
Becuase MemoryChunk class overrides #write_to to avoid calling #open for optimization.
What do you think?

https://github.com/fluent/fluentd/blob/master/lib/fluent/plugin/buffer/memory_chunk.rb#L84

[tagomoris]

#1172 and #1179 are pushed as pull-requests to solve this issue.